Infrared dynamic flame detector

ABSTRACT

A compact signal transmitter head infrared flame detector unit is mounted in a small opening in the wall of a fossil fuel fired furnace. The unit includes a preamplifier solid state circuit including an infrared sensor which is focused on the combustion zone of the furnace for sensing and transforming flame fluctuations into corresponding preamplified signal current fluctuations, which are conducted by a shielded cable to a transistorized main amplifier filter and rectifier circuit in a remote signal receiver terminal unit. The unit has the capability of distinguishing from the fluctuations inherent in the flame from the background radiation. An integrated control circuit is associated with the main amplifier circuit for digitally processing the signal fluctuations and automatically operating flame indicators and/or alarms as required by the condition of the flame combustion zone.

United States Patent Horn Sept. 2, 1975 INFRARED DYNAMIC FLAME DETECTOR Robert Horn. Richardson, Tex.

Forney Engineering Company, Carrollton. Tex.

Dec. 14, I973 [75] Inventor:

[73] Assignee:

[22] Filed:

Appl. No.: 425,039

Primary E.mntitwr-Archie R. Borchelt Attorney, Agent. or Firm-John P. De Luca; Marvin A. Naigur; John E Wilson [57] ABSTRACT A compact signal transmitter head infrared flame detector unit is mounted in a small opening in the wall of a fossil fuel fired furnace. The unit includes a preamplifier solid state circuit including an infrared sensor which is focused on the combustion zone of the furnace for sensing and transforming flame fluctuations into corresponding preamplified signal current fluctuations, which are conducted by a shielded cable to a transistorized main amplifier filter and rectifier circuit in a remote signal receiver terminal unit. The unit has the capability of distinguishing from the fluctuations inherent in the flame from the background radiation. An integrated control circuit is associated with the main amplifier circuit for digitally processing the signal fluctuations and automatically operating flame indicators and/0r alarms as required by the condition of the flame combustion zone.

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INFRARED DYNAMIC FLAME DETECTOR BACKGROUND OF THE INVENTION The present invention relates to an infrared flame detector system for monitoring fossil fuel fired furnaces and controlling indicators as well as the flow of fuel to selected burners in accordance with flame operation. For example, in case of flame failure, it is desirable that such failure be indicated and an alarm be operated and fuel flow be stopped as soon as possible to avoid flood ing and possible explosion in the furnace.

While the present invention is applicable to both coal and oil fired furnaces, it is particularly advantageous where used with pulverized coal fired burners. The instant invention also finds advantageous use in connection with gas fired furnaces provided with oil fired igniters, since the infrared detector can discriminate between the oil fired ignition and the gas fired combustion. In the case of coal fired burners, the infrared detector is able to see through the coal dust and smoke enveloping the combustion zone, which is not possible with ultra-violet flame detectors which require the flame sighting to take place relatively close to the flame.

The invention provides an infrared dynamic detector system for fuel fired furnaces, in which amplification of the sensed signal is accomplished by a small transistorizcd preamplifier circuit, and a remote solid state main amplifier circuit. The main amplifier circuit is housed in a signal receiver terminal unit that is remote with respect to the preamplifier circuit which is housed in a signal transmitter unit or head. The circuits in such units are connected by a two-conductor shielded cable extending a considerable distance from one unit to the other. The signal transmitter unit is relatively compact, and is provided with a small optical nipple that fits in an opening provided therefor in the furnace wall, substantially in line with the flame being monitored.

The small solid state infrared signal preamplifier circuit includes an infrared sensor (photosensitive) cell which is aimed at the combustion zone of the monitored flame, with such nipple being located on the head unit so as not to extend into the furnace. Thus, the cell is able to pick up infrared radiation emitted in the combustion zone of the furnace without damage through heat and sparks adjacent thereto, by seeing through any murky atmosphere around such zone. The infrared sensor transforms the flame fluctuations which are present in the combustion zone under surveillance by the cell into a fluctuation equivalent electrical signal that is preamplified by the transistor circuit in the transmitter unit for conduction through the long shielded cable to the main amplifier circuit in the remote signal receiver terminal unit.

The main amplifier circuit receives the signal fluctuations from the infrared sensor circuit. and further amplifies such fluctuations. The main amplifier circuit comprises a filter section responsive to an optimum frequency range of from 40 to 60 Hz in the light source frequency, since the primary combustion zone is very rich in such frequencies, and background frequencies fall outside of such range. The output signal db/light source frequence characteristic curve between 9 and 75 Hz rises gradually to a maximum of +2 db at 75 H7. (SUHz equals (J db). and then falls somewhat more slowly at higher H7. values of light source frequencies. Undesirable noise is thus eliminated. The main ampli fier circuit in combination with an integrated control circuit further amplifies and digitally-processes the amplified signal fluctuations for controlling flow of fuel to the flame burner being monitored in a flame safety and alarm system which, for example, operates an alarm indicator and shuts off flow of fuel to the burner automatically when such flame fails after a suitable delay.

In brief, the system of the present operation operates as follows: As the variable infrared radiation from the combustion or ignition zone strikes the photosensitive cell, the resistance of the cell changes with the intensity of the infrared source. Such variable resistance in conjunction with a constant current, generates a variable voltage which is amplified. The amplified variable voltage in turn, is converted to a variable current which is driven through the shielded cable to the main amplifier. In the main amplifier the variable current flows through a resistor, generating a variable voltage which is coupled through a capacitor to a first stage integrated amplifier. After such first stage of amplification, the AC signal of flame fluctuations is passed to an integrated filter circuit. After the signal is filtered, it is applied to a half-wave rectifier and integrator. The resultant signal (DC) is compared with a manually pre-set integrator signal (DC) of the background, and processed accordingly through time delay and digital circuits as required for controling the burner.

The terminal unit is provided for receiving the preamplified signals corresponding to the infrared fluctuations sensed by the photosensitive cell viewing the combustion zone in the furnace, from the detector head, which are transmitted through the elongated shielded cable for a considerable distance. Such terminal unit comprises four major sections consisting of the following: 1) an amplifier and filter section; 2) a rectifier section; 3) a signal level detector section; and 4) a time delay section. The first section comprises a circuit for further amplifying and filtering that part of the signal which is between 45 and cps, and rejects all other frequencies to some extent. The second section comprises a circuit for converting the AC output of the first section to a proportional DC level. The third section comprises a circuit which then compares the output of the second section with a pre-set limit to determine if a flame is present or not. The fourth section comprises an adjustable time delay circuit on flame out before the output signals F (FLAME) and F (NOT FLAME) from the circuit responds to the flame out. There is no delay when the flame is first detected.

The present invention provides a frequency response characteristic curve in the light source frequency range of 456() Hz which produces a minimum of undesirable noise, i.e., between -I and +1 -db (SOHZ equals 0 db as standard) in the amplified signal fluctuations. Since the signal transmitter unit or head is mounted in a small opening in the wall of the furnace, it does not suffer from the problems which affect prior sensors in which a lengthy sight tube containing an ultra-violet photosensitive cell, extends into the furnace for monitoring the flame. Such tubes often fail due to the intense heat, and also the severe thermal stress decreases the accuracy of the device. Thus, the present invention is particularly advantageous when used in coal fired furnaces.

In gas fired furnaces with oil fired ignitors, the present infrared detector can distinguish between the two. indicating when the igniter is in operation. This is not possible in the case of conventional ultra-violet detectors. Also, the invention can be quickly and easily serviced should that become necessary in the field by simply replacing one or more of the units or parts thereof.

SUMMARY OF THE INVENTION In accordance with an illustrative embodiment demontrating features and advantages of the present invention, there is provided an infrared dynamic flame detector for use with fossil fuel fired burners in a furnace formed from walls having port openings for receiving the burners. A signal transmitter unit is mounted in the openings including a preamplifier circuit having an infrared sensor provided with an optical nipple aimed at the combustion zone for transforming flame fluctuations thereof into corresponding preamplified signal fluctuations. A remote signal receiver unit is provided with a main signal amplifier circuit and a shielded cable connects the units for conducting signal fluctuations from the preamplifier circuit to the main amplifier circuit. An integrated control circuit is associated with the main amplifier circuit for digitally processing the amplified signal fluctuations.

BRIEF DESCRIPTION OF THE DRAWINGS The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiments in accordance with the present invention, when taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic view of an infrared dynamic detector system embodying the invention;

FIG. 2 is a view mainly in side elevation of a coal fired boiler provided with a flame detector head of the invention located in an opening in the furnace wall;

FIG. 3 is a block diagram of the system of the invention;

FIG. 4 is a circuit diagram of the entire system;

FIG. 5 is a characteristic frequency response curve of the output signal/light source frequency, provided by the main amplifier; and

FIG. 6 is a fragmentary view mainly in side elevation ofa gas fired burner having an oil fired ignitor provided with a flame detector head comprising the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. I, the infrared dynamic flame detector system is composed of a compact head unit 10, an elongated shielded cable 12, and a remote terminal unit 14 which may be located a considerable distance from the head unit 10. The head unit has a short tubular projection constituting an optical nipple 16 that fits a small corresponding opening in the furnace wall 18 of a boiler 20, for example, in which a burner 22 produces a flame 24. Preamplified signals, corresponding to the flame fluctuations, are generated and transmitted from the head 10 through cable 12 to the terminal unit 14 where such preamplified signals are further amplified and digitally processed for control and/or indication of the ON/OFF state of flame 24, as may be required.

As shown in FIG. 2, the line of sight 26 of the optical system in the short tube 10 or nipple 16 is parallel to burner 22, but within air register 28, and through the throat 30 of the boiler 20. The flame 24 is surrounded by a conical train 32 of smoke and coal dust, in a combustion zone 34, which is seen through the line of sight 26 when the burner 22 is in operation. Thus. the head 10 is protected from thermal damage while in use.

As shown in FIG. 3, the head 10 comprises a box which contains an infrared photosensitive cell 36 and a small solid state preamplifier circuit 38. The cell 36 is arranged with the optical system for exposure to radiation emitted from the flame under surveillance. The cell 36 and preamplifier circuit 38 transform the flame fluctuations, which are present in the combustion zone, into an equivalent electrical signal which is preamplified for transmission through the cable 12 to the terminal unit 14. The cable 12 is shielded and contains two insulated conductors. A main amplifier circuit 40 is provided in the terminal unit 14 as shown in FIG. 3, and may be located some distance from the detector head 10.

The terminal 14 utilizes integrated circuits and includes the following four major sections, 1 a main amplifier and filter section 40; 2) a rectifier section 42; 3) a signal level detector section 44; and 4) a time delay section 46 having output signal circuits with F designating FLAME, Fdesignating NOT FLAME and F, desinating instantaneous FLAME. The circuit of section 40 amplifies that part of the signal which is between 45 and 60 cps (Hz), and rejects all other frequencies to a certain extent. The circuit of section 42 converts the AC output of the amplifier filter section 40 to a proportional DC level; and the circuit of section 44 then compares the output of the rectifier section 44 with a preset limit to determine if a flame is present or not. The output of the level detector section 44 goes to the time delay section 46, the circuit of which provides an adjustable delay on flame out before the F (FLAME) and F(NOT FLAME) signals respond. There is no delay on the flame F, signal.

The response curve 50, FIG. 5, of the main amplifier filter section 40 rises to a maximum of +2 db output signal at about Hz of light source frequency, and then falls gradually thereafter, as shown. The output signal is plotted in db (50 Hz equals 0 db as standard). The primary combustion zone is very rich in such frequencies, whereas the tail end or background flame frequencies fall outside of the response curve 50. The optimum frequency range is selected from 45 to 60 Hz, and frequencies substantially above and below such range are cut off. This places the useful response on the front portion of the curve 50, which effectively eliminates undesirable noise.

As shown in FIG. 4, the photosensitive cell 36 is provided with pin type terminals A, B and C for quick assembly with and removal from corresponding sockets which are connected to the preamplifier circuit 38. Such circuit comprises a positive supply voltage lead 52, a negative lead 54, and a ground lead 56. Such leads are connected to corresponding leads in the shielded cable 12 by quick disconnect pin and socket connectors E, .I and H, respectively. Thus, the entire head 10 can be removed quickly for repair and/or re placement in the field. The cell 36 converts variable radiation to a variable resistance.

Resistor R5, capacitor C2 and diode D2 are connected to provide a constant voltage source to two constant current sources. Capacitor C2 provides filtering for the constant voltage source. Resistor R6 and transistor O4, along with such constant voltage source gencrate a constant current source for the cell 36. Likewise. resistor R4, transistor Q5 and resistor R2 are connected in series across leads 52 and 54 to provide a constant current source 'for biasing a Darlington connccted configuration of transistors Q1 and Q2, also connected across leads 52 and 54. Transistor Q3 and resistor R3 are connected in the circuit 38 to provide an impedance match between the cell circuit (high impcdance) and the output transistors (low impedance). A capacitor C1 couples only the AC component of the flame signal to the output transistors Q1, Q2.

In operation, as variable infrared strikes the cell 36, the resistance thereof changes with the intensity of the infrared source. The constant current and variable cell resistance generate a voltage at the terminal A of the cell 36. Such variable voltage is then coupled to output transistors Q1, Q2 by the operation of the circuit comprising transistor Q3, resistorRS and capacitor C1. Transistors Q1 and Q2 convert this variable voltage to a variable current signal which is driven through the shielded cable 12 to the main amplifier resistor R27.

The terminal unit 14 receives the output signal from the cable 12 through quick disconnect pin/socket connectors or terminals G and I, for repair and/or replacement of the unit 14 as may be necessary in the field. The main amplifier circuit is provided with a positive voltage lead 58 that is connected to the(+) terminal 60 of a single end 12 volt DC supply. Resistor R15 and diode D22 are connected in series across lead 58 and the ground lead 62, which also contains resistor R27, to provide a reference voltage for the operational amplifier, since the amplifier operates across the single and supply (-l-l2VDC). l

A capacitor C10 is connected inth'e circuit to provide an AC coupling for the first stage 64 of amplification of the received signal. Undesirable high'frequencies are shunted to ground by a capacitor C9 which is connected across resistor R27. The first stage 64 of the signal amplification circuit comprises a resistor R19, an integrated circuit IC2, a resistor R28, a capacitor C13, a resistor R22, adjustable resistor R11, and a capacitor C12. The voltage gain of the first stage 64 of amplification is controlled by adjustment of resistor R11. The main frequency filter circuit 66 comprisesa capacitor C5, a resistor R17, a resistor R16, a resistor R14, a capacitor C3, a capacitor C6, an integrated circuit 1C3, a capacitor C7, a resistor R18, a resistor R33, and 21 capacitor C44. The frequency filter circuit 66 attenuates the high and low fre uencies of the flame signal, preferably above and below a selected range of 45 to 60 Hz.

A half-wave rectifier-integrator and filter circuit 68 is provided in the unit 14, comprising an integrated circuit IC4. a diode D33. a diode D4, a resistor R21, a capacitor C8, a diode D5, a resistor R222, a capacitor C111, and a resistor R13. The circuit 68 rectifies the filtered flame signal and provides a DC voltage at the base of transistor Q22.

Transistor Q22 and resistor R21) are connected to provide an emitter follower circuit 70 for isolation between the rectifier integrator circuit 68 and an integrated comparator circuit 71 and connections to a meter M.

A voltage divider circuit 72 for background voltage generation is provided comprising a resistor R8, an adjust-able resistor R10, and a resistor R9. The desired background setting is obtained by adjusting resistor R10.

The comparator circuit 71 comprises an integrated circuit i-CS, a resistor R25 and a resistor R12. The comparator circuit 71 acts to compare the DC flame signal to the background signal, and generates an appropriate output to circuit 79.The pins 76, 78 provide quick disconnect means for the meter M from the unit 14 for repair calibration, and/or replacement.

An (1C1) integrated circuit 79 provides a driver for the signal through the following circuit stages including a time delay circuit 81). The circuit comprises, in addition to the integrated circuit 79, a diode D11, a resistor R7, an adjustable resistor R66, a resistor R55, a capacitor C11, a transistor Q11, a resistor R44. The 1C1 integrated circuits 82, 84 and 86, have three flame signal output circuits 88, and 92 for flame F, or no flame 1 on a delayed flame loss; and flame F, without a delay.

The variable current flowing through resistor R27 from the preamplifier circuit 38, generates a variable voltage across the resistor R27, which is coupled through capacitor C10 to the first stage of amplification 64. The AC signal is amplified, and passed to the filter section 66. After being filtered the signal is passed on to the half-wave rectifier and integrator section 68. The resultant DC signal is compared with the background setting by the comparator stage 71 and automatically processed accordingly through digital circuit 79, the time delay, circuit 80 and digital circuits 82, 84 and 86. I

As shown in FIG. 5, the main filter section 66 of FIG. 4, is used to attenuate the high and low frequencies above 60 Hz andvbelow 45 HZ of the frequency re sponse characteristic 50 of the infrared cell36, the preamplifier 38 andmain amplifier64 are utilized for optimum results. Since the best frequency range is from 45 to 60 Hz, the'front portion of the frequency response curve 50 is used, and frequencies substantially above 60 1-17. are cut off thus eliminating noise. Y

As shown in FIG. 6,'the head. 10 of the present inven tion is used in a, gas fired furnace 21 "provided with a openings in furnace wall 19, as does the optical nipple 16 of the head 10. The line of sight 17 of the sensor in this case looks at both the zone of the gas flame 29 and i that of the oil (igniter) flame27. The present infrared detector can discriminate between the oil fired ignition and the gas fired ignition, and thus determine whether the oil fired igniter is in operation. This is not possible with conventional ultra-violet flame detectors.

The present invention is also applicable to coal and oil fired boilers. However, it is particularly advantageous when used in a coal fired boiler because the use of infrared radiation is one of the only successful ways of flame detection in a coal fired boiler. Due to the smoke and dust created in a coal fired furnace, it is difficult if not impossible to detect the presence or absence of flame using other methods of detection. Heretofore, the principal method of flame detection required an ultra-violet sensor which in turn required a rather lengthy tube extending into the furnace, and with such long tube being close to the flame, it was sub jcct to severe thermal stresses which quickly render the optical system inaccurate. Also, the ultra-violet light, necessary for proper operation of the ultra-violet detector, was masked by the smoke and coal dust which are present in the furnace. The present invention does not suffer from such problems, since the head is substantially flush with furnace wall.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistant with the spirit and scope of the invention herein.

What is claimed is:

1. An infra-red dynamic flame'detector comprising: infra-red sensor means responsive to energy of a selected bandwith and providing an AC output signal in response thereto, means responsive to said AC signal for amplifying same when the frequency thereof falls between about 45 and 60 CPS and substantially rejecting all other signals. rectifier means for converting said amplified AC signal to a proportional DC level of the amplifier means output circuit means for comparing the DC output of the rectifier means with a selected limit value and delivering an appropriate output indicative thereof, time delay means responsive to said comparitor output for providing an output indicative of a flameout condition for a period longer than a selected delay of said time delay means, when the proportional DC signal falls below the selected limit whereby a flame momentarily affected by fluctuations in intensity may be detected. 1

2. The infra-red dynamic flame detector of claim 1 wherein said infra-red sensor includes an infra-red responsive cell and an optical nipple for coupling the infra-red energy of the flame thereto.

3. The infra-red dynamic flame detector of claim 2 further including a preamplifier for preamplifying the infra-red signals detected'by the cell through said nipple and delivering an output signal therefrom.

'4. The infra-red dynamic flame detector of claim 3 further including a remote receiver terminal unit for housing said amplifier means, rectifier means, signal detector means and time delay means and a conducting cable for coupling the output signals of said preamplifier to the amplifier of said remote receiver terminal unit.

5. The infra-red dynamic flame detector according to :laim 3 wherein said preamplifier includes requlated DC voltage supply means and amplifier means responaive to the output of the cell for converting the variable nfra-red radiation energy into a signal corresponding to a variable resistance, and output circuit means for delivering only the AC component of the signal corresponding to the variable resistance.

6. The infra-red dynamic detector according to claim 5 wherein said regulated DC voltage supply includes leads of opposite polarity, a resistor and a diode con nected in series across said leads, a capacitor connected in a parallel circuit with said resistor for filtering said constant voltage, a resistor and transistor connected in series across said positive and negative leads, means connecting the base of said transistor to the negative side of said capacitor, so that a constant current source is provided for the cell which converts variable infra-red radiation to the corresponding variable resistance, a resistor and a transistor connected in series across said leads for generating a constant current source, two output transistors connected in a Darlington configuration across said leads, and to said lastnamed transistor for biasing the latter configuration with such constant current output, a transistor and a resistor connected in series across the cell circuit and output transistors to provide an impedance match, and a capacitor coupling-only the AC component of the flame signal to the output transistors.

7. An infra-red dynamic flame detector according to claim 4 in which said cell is provided with a ground, positive and negative, leads each provided with quick disconnect terminal pin elements for mating with corresponding external socket elements, so that said cell can be easily removed and replaced.

8. An infra-red dynamic flame detector according to claim 4 including a housing for said optical nipple, preamplifier and cell and wherein said conducting cable is equipped with quick disconnect pin elements mating with corresponding circuit socket elements in said housing and remote terminal unit.

9, An infra-red dynamic flame detector according to claim 6 in which said cell resistance changes in accordance with fluctuations in the infra-red source intensity, which variable cell resistance together with the constant current generates a variable voltage at the positive terminal of said cell, such variable voltage then being coupled to the output transistors, thereby causing the latter to convert such voltage to variable current which constitutes the preamplified signal output, which is thereafter transmitted. 

1. An infra-red dynamic flame detector comprising: infra-red sensor means responsive to energy of a selected bandwith and providing an AC output signal in response thereto, means responsive to said AC signal for amplifying same when the frequency thereof falls between about 45 and 60 CPS and substantially rejecting all other signals, rectifier means for converting said amplified AC signal to a proportional DC level of the amplifier means output circuit means for comparing the DC output of the rectifier means with a selected limit value and delivering an appropriate output indicative thereof, time delay means responsive to said comparitor output for providing an output indicative of a flameout condition for a period longer than a selected delay of said time delay means, when the proportional DC signal falls below the selected limit whereby a flame momentarily affected by fluctuations in intensity may be detected.
 2. The infra-red dynamic flame detector of claim 1 wherein said infra-red sensor includes an infra-red responsive cell and an optical nipple for coupling the infra-red energy of the flame thereto.
 3. The infra-red dynamic flame detector of claim 2 further including a preamplifier for preamplifying the infra-red signals detected by the cell through said nipple and delivering an output signal therefrom.
 4. The infra-red dynamic flame detector of claim 3 further including a remote receiver terminal unit for housing said amplifier means, rectifier means, signal detector means and time delay means and a conducting cable for coupling the output signals of said preamplifier to the amplifier of said remote receiver terminal unit.
 5. The infra-red dynamic flame detector according to claim 3 wherein said preamplifier includes requlated DC voltage supply means and amplifier means responsive to the output of the cell for converting the variable infra-red radiation energy into a signal corresponding to a variable resistance, and output circuit means for delivering only the AC component of the signal corresponding to the variable resistance.
 6. The infra-red dynamic detector according to claim 5 wherein said regulated DC voltage supply includes leads of opposite polarity, a resistor and a diode connected in series across said leads, a capacitor connected in a parallel circuit with said resistor for filtering said constant voltage, a resistor and transistor connected in series across said positive and negative leads, means connecting the base of said transistor to the negative side of said capacitor, so that a constant current source is provided for the cell which converts variable infra-red radiation to the corresponding variable resistance, a resistor and a transistor connected in series across said leads for generating a constant current source, two output transistors connected in a Darlington configuration across said leads, and to said last-named transistor for biasing the latter configuration with such constant current output, a transistor and a resistor connected in series across the cell circuit and output transistors to provide an impedance match, and a capacitor coupling only the AC component of the flame signal to the output transistors.
 7. An infra-red dynamic flame detector according to claim 4 in which said cell is provided with a ground, positive and negative, leads each provided with quick disconnect terminal pin elements for mating with corresponding external socket elements, so that said cell can be easily removed and replaced.
 8. An infra-red dynamic flame detector according to claim 4 including a housing for said optical nipple, preamplifier and cell and wherein said conducting cable is equipped with quick disconnect pin elements mating with corresponding circuit socket elements in said housing and remote terminal unit.
 9. An infra-red dynamic flame detector according to claim 6 in which said cell resistance changes in accordance with fluctuations in the infra-red source intensity, which variable cell resistance together with the constant current generates a variable voltage at the positive terminal of said cell, such variable voltage then being coupled to the output transistors, thereby causing the latter to convert such voltage to variable current which constitutes the preamplified signal output, which is thereafter transmitted. 